The selectivity of a desired chemical reaction is defined as the ratio of desired to both desired and undesired products made in the presence of a catalyst that may support both the desired and undesired chemical reactions. As such, improving selectivity to the desired product is of substantial value. For example, the elimination of, or reduction of carbon monoxide from a gas stream containing carbon monoxide, hydrogen and water vapor without significant consumption of hydrogen can be of value in numerous situations where the hydrogen will subsequently be used. One such example is in a fuel cell where hydrogen provides the fuel but carbon monoxide is a poison.
There are multiple fuel cell designs of which one is a Proton Exchange Membrane fuel cell (PEMFC). While the present invention finds utility with respect to PEMFC's, and such use is described herein, the present invention should not be considered to be limited in such regard. The present invention also finds utility in other applications where improved selectivity is desirable or, more fundamentally, where at least one chemical reaction is desired and at least one chemical reaction is undesired in the presence of a catalyst that may support both the desired and undesired chemical reactions.
A PEMFC includes an anode side and a cathode side separated by an electrolyte that acts as a membrane. On the anode side, hydrogen is introduced and each hydrogen molecule is split into two positively charged hydrogen ions and two electrons. Simultaneously on the cathode side, oxygen molecules are introduced and react with the positively charged hydrogen ions transported through the membrane. The electrolyte is treated to conduct only positively charged ions, pulling them through the electrolyte. The electrons released in the splitting of the hydrogen molecule are conducted through an anode on the anode side via an external circuit to a cathode on the cathode side where the hydrogen ions and oxygen combine to form water.
Hydrogen is the fuel used to operate a fuel cell and must be generated, e.g. concentrated or released from a molecule containing hydrogen, as hydrogen is not available in a natural form suitable for use as a fuel. One source of hydrogen is fossil fuel, such as gasoline, that is reformed to release the hydrogen contained therein. Gasoline is particularly desirable as a hydrogen source when the fuel cell is to be used as a power plant in a non-stationary item such as an automobile. A problem with obtaining hydrogen from gasoline, however, is that in the reformation process hydrogen is generated in combination with other gases such as carbon monoxide.
Carbon monoxide poisons the anode within a PEMFC potentially rendering the fuel cell less efficient or inoperative. Therefore, carbon monoxide must be removed from, or substantially reduced in, the gas stream containing the hydrogen prior to the gas stream being introduced into the PEMFC.
One method of reducing the carbon monoxide in a gas stream containing carbon monoxide (CO), hydrogen (H2) and water vapor (H2O) is to convert it to carbon dioxide (CO2) and hydrogen using a catalyst (employing the water gas shift reaction CO+H2O=CO2+H2). To accomplish this, low operational catalyst temperatures are preferred, since at high temperatures additional carbon dioxide can result in the net production of carbon monoxide. High operational catalyst temperatures, on the other hand, are problematic in that the hydrogen tends to combine with either the carbon monoxide, or carbon dioxide, if present, to form methane (CH4) and water in a process referred to as methanation (CO+3H2=CH4+H2O, or CO2+4H2=CH4+2H2O). For each molecule of methane formed by methanation, the available hydrogen for the fuel cell is reduced. It is, therefore, desirable to employ a method that reduces the concentration of carbon monoxide without simultaneously reducing the hydrogen present, or at least minimizing the consumption of hydrogen by the methanation process.
As is known to one skilled in the relevant art, the conversion of natural gas into a mixture of H2 and CO, known as synthesis gas or syngas, is an important intermediate step in many existing and emerging energy conversion technologies. Currently, most syngas is produced by steam reforming of hydrocarbons such as methane. This endothermic reaction is generally represented by the following equation:CH4+H2O→CO+3 H2 (ΔH=+206 kJ/mole)Methane is contacted with steam over a heated catalyst at high pressures and temperatures to produce a high hydrogen content syngas. However, steam reforming requires large heat exchange reactors, demanding large initial investment costs. Furthermore, stringent heat balance requirements in the steam reforming process make scaling these reactors to smaller sizes extremely difficult.
Alternatively, catalytic partial oxidation (CPOx) is a relatively inexpensive alternative for syngas generation. The CPOx process is generally represented by the following equation:CH4+½O2→CO+2 H2 (ΔH=−38 kJ/mole)A hydrocarbon such as methane is reacted with oxygen over a catalyst bed to yield syngas with H2 to CO molar ratio near 2. Examples of recent publications in this area are those of D. A. Hickman and L. D. Schmidt; AICHE Journal, 39 (1993) 1164 which discloses that near complete conversion of methane to mostly hydrogen and carbon monoxide could be achieved at reaction times as short as 1 millisecond thereby promising dramatic reduction in reactor size and complexity, as compared to existing syngas production technologies. However, to avoid the energy losses associated with the compression of hot, high hydrogen content gases, the CPOx reactor should be operated at pressures in the range of 0.5 to 4 MPa compatible with downstream processes.
Surprisingly, despite much interest over the last decade, only very limited work has been reported on high-pressure CPOx of methane and such work is not directly transferable to practical applications of a CPOx process. The properties of the catalytic systems discussed in the prior art promote liberating excessive amounts of heat when operating the CPOx process under elevated pressures, leading to overheating of the reactor. This problem in the prior art can be mitigated by enhancing the mass and heat transfer rates within the reactor. It is, therefore, desirable to employ a method that promotes steady operation of a CPOx reactor under the high-pressure conditions wherein the mass and heat transfer rates within the reactor are enhanced.
Based on the foregoing, it is an objective of the present invention to develop a method for improving the selectivity of a desired chemical reaction over an undesired chemical reaction, in the presence of a catalyst that may support both the desired and undesired chemical reactions. Another object of the present invention is to develop a method for the removal of carbon monoxide from a gas stream comprising carbon monoxide, hydrogen and water vapor wherein the consumption of hydrogen therein is minimized thereby resulting in a gas stream having a higher concentration of hydrogen than would otherwise be obtained. Another object of the present invention is to develop a method that promotes steady operation of a CPOx reactor under the high-pressure conditions wherein the mass and heat transfer rates within the reactor are enhanced. Another object of the present invention is to develop a method for the improved production of syngas.